Field of the Invention
The present invention relates to a method for determining the moisture content of a resin layer on a carrier board and the use of an NIR (Near Infrared) detector for determining the moisture content of a resin layer on a carrier board.
Description of Related Art
Wooden boards as carrier materials are used in different areas and are widespread. Thus, wooden boards are known amongst others for their use as floor panels, for instance in form of laminate floors, as insulation boards for internal and external area or also as wall panels. Such boards are usually made from wood fibers, wood ships or strands. In case of laminate floors, for instance HDF boards (HDF=fiber board with increased wall density) made of wood fibers are used with diverse decors and protective layers applied thereon.
The use of the so called overlay papers as protective or wear resistant layer is for instance known. Said overlay papers are thin papers typically impregnated with a melamine resin, wherein wear resistant particles as for instance corundum particles are mixed into the resin of the overlay in order to increase the wear resistance of the laminate or the wooden board. Besides the use of said overlay papers as wear resistant layer in another variant a powder is applied to the surface of the board, which comprises for instance natural or synthetic fibers, wear resistant particles and binder. Also in this case, the wear resistance and stability of the surface of the board is increased by using in particular wear resistant particles.
Besides the above-mentioned wear resistant layers in form of an overlay paper or a powder with wear resistant particles, the application of a liquid resin as protective layer is also known, in which also wear resistant particles as for instance corundum particles or glass balls can be contained. Such a liquid resin layer is also known as liquid overlay (EP 2 338 693 A1).
When producing wooden boards by using a liquid overlay an important criteria for quality insurance is the determination of the moisture during the coating process. When coating wooden boards, as for instance wooden boards provided with a printed décor layer, with water containing duroplastic resins, as for example urea or melamine resins, the water of the resin has to be reduced to a defined percentage by convection and/or a radiation, for instance by means of infrared (IR) and/or near infrared (NIR) radiation. The water to be removed stems on the one hand from the aqueous resin solution and on the other hand additional water results from the curing of the resin due to the condensation reaction. This water content has to be in a relative small range, since otherwise quality problems can occur during processing or in the final product.
The basic problem which arises during moisture determination of boards with liquid structure is the relative unfavourable ratio between applied resin water amount and the total weight of the coated board. This ratio is about 1:30 to 1:50 that means the applied liquid overlay layer is provided in very thin layers. Due to the percentage of the liquid overlay, which is small in respect to the weight, it is difficult to determine the moisture reduction for instance gravimetrically. Such a method is used for instance when determining the residual moisture of papers impregnated with thermosetting resins. There, the ratio between carrier (paper) and resin is about 1:1.
The determination of the moisture of a resin layer, which is applied as liquid overlay on a wooden board, using the so called Darr sample is also not possible. The Darr sample method is the most accurate method for determining wood moisture, wherein a sample is dried at 103+/−2° C. for 24 hours in a drying oven. The wood moisture is defined as the ratio of the weight of the water contained in the wood to the weight of the absolute dry wood (Darr weight). The wood moisture is therefore a ratio between wet weight and dry weight (Darr weight).
When using the Darr probe for a wooden board coated with a liquid overlay not only the moisture of the liquid overlay coating would be determined, but also the moisture of the wooden board, so that only a value of the total moisture of board and coating would be obtained. The Darr sample or the Darr method would also not allow a short-term change or interference in the drying process within the production line due to the drying process of 24 hours. Furthermore, such a moisture determination can also only be carried out using a final coated board after leaving the production line and due to this reason allows thus also no interference into the production process.
Also the possibility of sealing the wooden board, more exactly, the wooden carrier board by a water non-permeable foil would not be suitable for solving this problem, since the coating system would let pass moisture rising from the HWS carrier from the board.
The technical object of the present invention is thus to provide a method, which allows the determination of the moisture content of a resin layer, for instance a liquid overlay, provided on a wooden board which allows a sufficiently exact determination of the moisture and whereby quality deficiencies, which can be addressed to the moisture content of the resin layer, can be turned off.